One form of an extrusion head for the production of a tubular multilayer preform of softened thermoplastic material, with at least one translucent strip extending in the extrusion direction, which strip can serve for example for visually checking the content of a container produced from the preform, comprises at least two feed paths for feeding plasticised plastic material from a plurality of extruders to an annular passage arrangement. Arranged downstream of the annular passage arrangement is an extrusion nozzle having an annular slot or gap communicating with the annular passage arrangement, possibly with the interposition of an annular storage space. For the sake of brevity and convenience the nozzle having an annular gap or slot will be referred to hereinafter as an annular gap nozzle. The extrusion head further has at least one flow passage bore which opens into the annular passage arrangement, for introducing thereinto the translucent material for forming the viewing strip. In relation to such a piece of equipment reference may be made for example to WO 98/08668 describing an apparatus including an extrusion head of the above-indicated kind, for producing a drum-like container of plastic material with a viewing strip of translucent plastic material, which extends in the axial direction of the container in the wall thereof.
In quite general terms the production of plastic containers using an extrusion blow molding process, with a viewing strip extending in the longitudinal direction thereof, is a known procedure, more specifically both in relation to containers of a single-layer structure and also containers of a multilayer structure. In the case of single-layer containers produced by an extrusion blow molding procedure, a strip of translucent material or a material of a different color from the rest of the container is usually introduced in the extrusion head by way of a flow passage bore extending transversely with respect to the direction of extrusion of the container material through the extrusion head. That manner of introducing the translucent material for the viewing strip involves dividing the extrudate to open it in the extrusion head, thereby producing separate flow portions. Downstream of the location at which the viewing strip material is introduced into the extrusion head the flow portions are brought back together again, and the materials are welded together in a molten condition.
The axially extending weld seams which are produced in that way between the various flow portions of the extrudate are weak points in the finished container, but these can be perfectly well accepted when dealing with small and light packs.
However it is not only in relation to small containers that a visual check on the level of the contents in the container is often desirable. Such a need to check the level of the container content may arise for example in relation to intermediate bulk containers, referred to as IBCs, and drums.
Checking the filling level of the material in the container does not give rise to any problem if the containers are made from a more or less translucent material. Often however it is necessary, as in WO 98/08668, for such drum containers to be provided with a coloration effect which serves for example to afford protection from UV radiation or which is intended to make the drum electrically conductive as protection from electrostatic charges. Such containers are usually produced with a multilayer wall, in which case it is only the outer, generally thinner layer that has such a coloration effect.
In order to ensure visual checking of the level of the contents of such drums, WO 98/08668 provides that a viewing strip of translucent material is disposed only in the outer colored layers of the co-extruded container. This has the advantage in particular that there is no need to divide the extrudate into separate parts, in the region of the inner layer of the container, which imparts stability to the container.
WO 98/08668 involves the use of a storage head with an annular storage space which enlarges in a funnel shape. The piston of the storage head has three annular distributors which are each connected to a respective extruder, involving different molten plastic materials. The annular distributors communicate with an annular passage which is disposed upstream of the nozzle and more specifically the nozzle gap, with the extrudates being brought together in the annular passage. The translucent molten plastic material is fed into that annular passage by way of a flow channel bore, below the mouth opening of all the annular dividers, more specifically in such a way that the viewing strip of translucent material passes through only the two outer layers of the total of three layers making up the co-extrudate. Those two outermost layers comprise an outer colored layer and an inner layer of recycled material. The feed of translucent material occurs by way of a flow body which extends at least as far as the molten plastic material forming the inner layer of the preform that is to constitute the finished container.
It will be noted that the above-described arrangement suffers from the disadvantage that it is not possible to exactly fix the layer boundary of the translucent viewing strip in relation to the three-layer structure of the extrudate. In order to ensure that the molten translucent plastic material passes completely through the two outer layers, it is necessary under some circumstances for the flow body to be so positioned, directed radially inwardly in relation to the longitudinal axis of the extrusion head, that the extrudate forming the inner layer also flows around the flow body, even if only to a slight degree. It will be appreciated that this signifies a reduction in the wall thickness of the inner layer, which is closed in itself, in that region, and such a reduction in wall thickness thus entails a loss of stability of the container overall.
If the flow body does not extend entirely through both outer layers, the quality of the viewing strip suffers as a consequence. It is scarcely possible, or is a rather random matter, for the flow body to be caused to extend exactly into the plane of separation between the inner and outer layers. The above-described apparatus configuration is particularly disadvantageous if the thickness of the various layers of the preform is to be altered or varied. In that case it is necessary to adapt the depth of penetration of the flow body into the material according to the respective circumstances involved.